1. Field of the Invention
The present invention relates to a discharge-pumped excimer laser device having a cross-flow fan whose rotatable shaft is rotatably supported by magnetic bearings, and more particularly to a discharge-pumped excimer laser device with an improved layout for magnetic bearings and improved protective bearings.
2. Description of the Related Art
FIG. 1 of the accompanying drawings schematically shows a conventional excimer laser device. As shown in FIG. 1, the conventional excimer laser device has a casing 101 filled with a laser gas, a preliminary ionizing electrode (not shown) disposed in the casing 101 for preliminarily ionizing the laser gas, and a pair of main discharge electrodes 102 disposed in the casing 101 for producing an electric discharge to make it possible to oscillate a laser beam. The casing 101 also houses therein a cross-flow fan 103 for producing a high-speed gas flow between the main discharge electrodes 102.
The cross-flow fan 103 has a rotatable shaft 104 projecting from opposite ends thereof and rotatably supported in a non-contact manner by a plurality of radial magnetic bearings 106, 107 disposed on opposite sides of the casing 101 and an axial magnetic bearing 108 disposed near the radial magnetic bearing 106. The rotatable shaft 104 can be rotated by an induction motor 109 connected to an end thereof near the radial magnetic bearing 107. The casing 101 has a pair of windows 105 on its opposite ends for emitting the laser beam out of the casing 101.
When the radial magnetic bearings 106, 107 are not in operation, the rotatable shaft 104 is supported by protective bearings 110, 111 that are disposed respectively on the shaft end near the motor 109 and on the shaft end near the radial magnetic bearing 106. The protective bearings 110, 111 cannot use a general lubricant for the purpose of preventing the laser gas from being contaminated. The protective bearings 110, 111 are in the form of rolling bearings each comprising special self-lubricated balls that are highly resistant to corrosion and inner and outer races of stainless steel.
When a high voltage is applied between the main discharge electrodes 102, an electric discharge occurs therebetween to generate a laser beam. The generated laser beam is emitted through the windows 105 out of the casing 101. When the electric discharge occurs, the laser gas between the main discharge electrodes 102 is deteriorated and its discharge characteristics are impaired to the extent that no repetitive discharge pumping will be possible. To avoid this shortcoming, the cross-flow fan 103 is operated to circulate the laser gas in the casing 101 to generate a high-speed laser gas flow between the main discharge electrodes 102. Specifically, the laser gas between the main discharge electrodes 102 is replaced each time an electric discharge occurs therebetween for thereby performing stable repetitive pumping.
In the above conventional excimer laser device, however, the cross-flow fan 103 vibrates relatively largely during operation, tending to cause optical components (not shown) of the excimer laser device to have their optical axes displaced, imposing adverse effects on the properties of the laser beam. Specifically, when the excimer laser device is in operation, the laser gas in the casing 101 is pressurized to a pressure ranging from 1 to 3 kg/cm2 while the cross-flow fan 103 is rotating. Therefore, the cross-flow fan 103 needs a large drive power, and hence the motor 109 is required to be large in size. The motor 109 applies a rotational drive power to the cross-flow fan, and produces a radial magnetic attractive force which produces vibrations because of an eccentric positional error between its rotor and stator due to assembling errors and machining errors. Inasmuch as the radial magnetic attractive force is greater in proportion to the surface area of the rotor of the motor 109, the vibrations caused by the radial magnetic attractive force also become greater if the motor 109 is greater in size.
In recent years, there is a demand for discharge-pumped excimer laser devices to produce a high laser beam output by way of highly repetitive pumping. To achieve the highly repetitive pumping, the laser gas between the main discharge electrodes 102 needs to be replaced in a shorter period of time, and hence the laser gas flow generated by the cross-flow fan 103 needs to be higher in speed. The motor 109 needs to be large in size in order to rotate the cross-flow fan 103 at a higher speed. If the motor 109 becomes larger in size, the radial magnetic attractive force produced by the motor 109 is also larger in magnitude. Thus, the motor 109 produces larger vibrations, which make it difficult for the motor 109 to rotate at a higher speed. As a result, the discharge-pumped excimer laser device is unable to carry out stable highly repetitive pumping.
The protective bearings 110, 111 are positioned on the shaft ends where dust particles produced in the casing 101 during operation find it difficult to reach because such dust particles would otherwise enter rolling surfaces of the protective bearings 110, 111 to obstruct rotation thereof. With the protective bearings 110, 111 positioned on the shaft ends, however, when the rotatable shaft 104 of the cross-flow fan 103 is supported by the protective bearings 110, 111 while the radial magnetic bearings 106, 107 are not in operation, e.g., while the discharge-pumped excimer laser device is not in operation or is being shipped, the inter-bearing span or distance between the protective bearings 110, 111 is longer than when the rotatable shaft 104 is supported by the radial magnetic bearings 106, 107.
As a result, the static deflection of the rotatable shaft 104 supported by the protective bearings 110, 111 increases. Consequently, an air gap around the rotatable shaft 104 needs to be increased so as to prevent the outer circumferential surfaces of the rotatable shaft 104 at the radial magnetic bearings 106, 107 and the motor 109 from physically contacting inner casing surfaces. One problem with the increased air gap is that it reduces the operating forces of the radial magnetic bearings 106, 107. Specifically, as the air gap becomes greater, larger magnetic bearings are required. Since the operating forces of magnetic bearings are generally lowered in proportion to the square of the air gap, if the air gap is increased twice, then magnetic bearings that are four times greater in size will be required.
If the rotatable shaft 104 needs to be supported by the protective bearings 110, 111 due to a failure of the radial magnetic bearings 106, 107, then the critical speed of the rotatable shaft 104 is reduced as the inter-bearing span becomes longer than when the rotatable shaft 104 is supported by the radial magnetic bearings 106, 107. When the rotatable shaft 104 is supported by the protective bearings 110, 111, therefore, it suffers violent vibrations upon rotation, displacing the optical axes of the optical components of the discharge-pumped excimer laser device. For restarting the discharge-pumped excimer laser device, therefore, the optical axes are required to be adjusted again. Accordingly, the discharge-pumped excimer laser device cannot quickly be restarted.
The self-lubricated balls of the protective bearings 110, 111 have a relatively low allowable rotational speed and allowable load because they have a problem as to their mechanical strength. If the cross-flow fan 103 rotates at higher speeds and the motor 109 becomes larger in size and hence the rotatable shaft 104 becomes larger in size, then the protective bearings 110, 111 cannot be used due to the insufficient mechanical strength thereof.
FIG. 2 of the accompanying drawings shows the conventional cross-flow fan 103. As shown in FIG. 2, the conventional cross-flow fan 103 comprises a plurality of parallel blades 103a, a pair of ring plates 103b attached to opposite ends of the blades 103a, and a pair of ring plates 103c disposed between the ring plates 103b at spaced intervals in the axial direction of the rotatable shaft 104. The ring plates 103c have attachment holes or recesses defined therein near their outer circumferential edges for attachment of the blades 103a. The ring plates 103b on the opposite ends of the blades 103a also have attachment holes or recesses defined therein near their outer circumferential edges for attachment of the blades 103a, and attachment bosses at their inner circumferential edges for attachment of the rotatable shaft 104. For assembling the cross-flow fan 103, the blades 103a are inserted through the attachment holes or recesses axially along the rotatable shaft 104, and the outer circumferential edges of all the ring plates 103b, 103c are crimped to set the blades 103a in place.
The rotatable shaft 104 is installed in order to support the cross-flow fan 103 rotatably and impart the rotational drive power to the cross-flow fan 103. The rotatable shaft 104 extends axially through the cross-flow fan 103 and projects from the ring plates 103b at the opposite ends. Displacement sensor target and electromagnet targets of the magnetic bearings, and the motor rotor are fixed to the projecting ends of the rotatable shaft 104. The rotatable shaft 104 extends axially through the cross-flow fan 103 because the cage-shaped structure composed of the blades 103a and the ring plates 103b, 103c is low in mechanical strength, and the displacement sensor targets, the electromagnet targets, and the motor rotor as mounted on the opposite ends of the cross-flow fan 103 would deform the cross-flow fan 103.
The cross-flow fan 103 and the rotatable shaft 104 are assembled together by inserting the rotatable shaft 104 axially through the cross-flow fan 103 and the attachment bosses of the ring plates 103b are fixed to the rotatable shaft 104 by setscrews 103d. 
The cross-flow fan 103 shown in FIG. 2 is problematic in that the setscrews 103d are liable to work loose due to vibrations applied while the discharge-pumped excimer laser device is in operation or is being shipped. If the cross-flow fan 103 is made of aluminum, then when the rotatable shaft 104 undergoes a temperature cycle to increase its temperature during discharge pumping, the setscrews 103d may possibly work loose. When the setscrews 103 are loosened, the cross-flow fan 103 is displaced axially, failing to produce a desired gas flow between the main discharge electrodes 102. If no desired gas flow is produced, then the discharge-pumped excimer laser device is incapable of stable discharge pumping. The rotatable shaft 104 is also possibly displaced radially in gaps between itself and the attachment bosses, tending to change unbalancing forces on the rotatable shaft 104. When unbalancing forces on the rotatable shaft 104 are changed, large vibrations are produced to displace the optical axes of the optical components of the discharge-pumped excimer laser device, thus adversely affecting the laser beam output.
It is therefore an object of the present invention to provide a discharge-pumped excimer laser device which will solve the problems of the conventional discharge-pumped excimer laser device and has a cross-flow fan that causes relatively small vibrations and can rotate at a high speed.
To achieve the above object, there is provided in accordance with the present invention a discharge-pumped excimer laser device comprising a casing filled with a laser gas, a pair of main discharge electrodes disposed in the casing for producing an electric discharge to discharge-pump the laser gas to emit a laser beam, a cross-flow fan for producing a high-speed laser gas flow between the main discharge electrodes, the cross-flow fan having a rotatable shaft projecting from opposite ends thereof, magnetic bearings, the rotatable shaft being rotatably supported in a non-contact manner by the bearings, protective bearings for supporting the rotatable shaft when the magnetic bearings are not in operation, and a motor for actuating the cross-flow fan, the magnetic bearings including radial magnetic bearings disposed respectively on the opposite ends of the cross-flow fan, the motor being disposed on an end of the rotatable shaft near one of the radial magnetic bearings, the one of the radial magnetic bearings having a bearing rigidity greater than the bearing rigidity of the radial magnetic bearing which is disposed remotely from the motor.
With the above arrangement, since the bearing rigidity of the radial magnetic bearing near the motor is greater than the bearing rigidity of the radial magnetic bearing which is disposed remotely from the motor, vibrations caused by radial magnetic attractive forces of the motor are effectively suppressed by the radial magnetic bearing disposed near the motor, and vibrations caused by an unbalanced state due to a misalignment between the center of rotation of the rotatable shaft and the center of gravity of the rotatable shaft are also suppressed by the radial magnetic bearing whose bearing rigidity is greater than the bearing rigidity of the other radial magnetic bearing. Therefore, the cross-flow fan causes reduced vibrations and can be rotated at a high speed, and hence the discharge-pumped excimer laser device is capable of repetitive discharge pumping and can emit a laser beam of stable characteristics.
The radial magnetic bearings have respective electromagnets having respective cores, the core of the electromagnet of the one of the radial magnetic bearings having a cross-sectional area greater than the cross-sectional area of the core of the electromagnet of the radial magnetic bearing which is disposed remotely from the motor, whereby the bearing rigidity of the one of the radial magnetic bearings is greater than the bearing rigidity of the radial magnetic bearing which is disposed remotely from the motor.
The radial magnetic bearings have respective electromagnets and respective electromagnet targets, and the dimension of a gap between the electromagnet and the electromagnet target of the one of the radial magnetic bearings is smaller than the dimension of a gap between the electromagnet and the electromagnet target of the radial magnetic bearing which is disposed remotely from the motor, whereby the bearing rigidity of the one of the radial magnetic bearings is greater than the bearing rigidity of the radial magnetic bearing which is disposed remotely from the motor.
The radial magnetic bearings have respective electromagnets including respective coils, and the number of turns of the coil of the electromagnet of the one of the radial magnetic bearings is greater than the number of turns of the coil of the electromagnet of the radial magnetic bearing which is disposed remotely from the motor, whereby the bearing rigidity of the one of the radial magnetic bearings is greater than the bearing rigidity of the radial magnetic bearing which is disposed remotely from the motor.
The magnetic bearings include another radial magnetic bearing disposed on a shaft end of the motor.
The rotatable shaft is rotatably supported by the radial magnetic bearings disposed on the opposite ends of the cross-flow fan and the radial magnetic bearing disposed on the shaft end of the motor. The motor is disposed outwardly of the span between the radial magnetic bearings disposed on respectively on the opposite ends of the cross-flow fan, and applies a rotational drive power to the cross-flow fan. With the motor being thus positioned, vibrations caused by radial magnetic attractive forces of the motor are suppressed by the radial magnetic bearing disposed on the shaft end of the motor. The cross-flow fan causes reduced vibrations and can be rotated at a high speed, so that the discharge-pumped excimer laser device is capable of repetitive discharge pumping and can emit a laser beam of stable characteristics.
According to the present invention, there is also provided a discharge-pumped excimer laser device comprising a casing filled with a laser gas, a pair of main discharge electrodes disposed in the casing for producing an electric discharge to discharge-pump the laser gas to emit a laser beam, a cross-flow fan for producing a high-speed laser gas flow between the main discharge electrodes, the cross-flow fan having a rotatable shaft projecting from opposite ends thereof, magnetic bearings, the rotatable shaft being rotatably supported in a non-contact manner by the bearings, the magnetic bearings including radial magnetic bearings disposed respectively on the opposite ends of the cross-flow fan, protective bearings for supporting the rotatable shaft when the magnetic bearings are not in operation, a motor for actuating the cross-flow fan, and a laser gas inlet passage for introducing the laser gas, from which dust particles are removed, into ends, remote from the cross-flow fan, of the radial magnetic bearings disposed respectively on the opposite ends of the cross-flow fan and the motor, or a differential pressure generating mechanism disposed between gas flow paths interconnecting the casing and the magnetic bearings, the protective bearings including protective bearings disposed respectively near the radial magnetic bearings on the opposite ends of the cross-flow fan.
With the above arrangement, because the laser gas inlet passage is provided for introducing the laser gas, from which dust particles are removed, into ends, remote from the cross-flow fan, of the radial magnetic bearings disposed respectively on the opposite ends of the cross-flow fan and the motor, or the differential pressure generating mechanism is disposed between gas flow paths interconnecting the casing and the magnetic bearings, dust particles produced in the casing while the discharge-pumped excimer laser device is in operation do not enter the magnetic bearings and the motor. Consequently, the protective bearings do not need to be disposed on the ends of the rotatable shaft, but can be disposed near the radial magnetic bearings. The inter-bearing span of the rotatable shaft when it is supported by the radial magnetic bearings is substantially the same as the inter-bearing span of the rotatable shaft when it is supported by the protective bearings, and as a result, the static deflection of the rotatable shaft remains substantially the same regardless of whether the rotatable shaft is supported by the radial magnetic bearings or the protective bearings. Consequently, an air gap between the outer circumferential surfaces of the rotatable shaft at the radial magnetic bearings and the motor and inner casing surfaces can be reduced. Thus, the magnetic bearings and the motor can be reduced in size.
Even if the rotatable shaft needs to be supported by the protective bearings due to a failure of the radial magnetic bearings, the critical speed of the rotatable shaft does not change greatly, and hence vibrations of the rotatable shaft are reduced upon its rotation. Therefore, other components and peripheral devices of the discharge-pumped excimer laser device are not adversely affected by the vibrations, and hence discharge-pumped excimer laser device can quickly be restarted.
The protective bearings comprise rolling bearings each comprising rolling members, an inner race, and an outer race, at least one of the rolling members, the inner race, and the outer race being made of alumina ceramics or zirconia ceramics.
The alumina ceramics or zirconia ceramics, which at least one of the rolling members, the inner race, and the outer race is made of, is corrosion-resistant to the laser gas and has a large mechanical strength. The protective bearings made of the alumina ceramics or zirconia ceramics have a long service life and may be replaced at long intervals. Even when the rotational speed of the cross-flow fan is higher or the motor is larger in size, posing a larger load on the protective bearings, the protective bearings can operate effectively.
The protective bearings comprise sliding bearings each made of alumina ceramics, zirconia ceramics, polytetrafluoroethylene, or a composite material thereof.
The sliding bearings each made of alumina ceramics, zirconia ceramics, polytetrafluoroethylene, or a composite material thereof are of a structure having less gas traps and can be manufactured relatively inexpensively.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.